In the preparation of high purity zirconium fluoride by the process known as chemical vapor purification (CVP), the presence of zirconium-oxygen bonds have been observed in the product of the process. It appears that the oxygen is substituting for F in the crystal structure of the ZrF.sub.4 sample, thus carrying undesirable oxygen-bearing species into the materials prepared using the ZrF.sub.4.
The potential sources of the oxygen are very limited, but for completeness, could include the chlorine, the argon, the reactor, and/or the hydrogen fluoride used in the CVP process.
The argon used in the process has been treated by passing it over titanium metal heated to 700.degree. C., a technique well known to remove oxygen to partial pressures 10.sup.-20 atmospheres [Data Sheet on Model 2A-100SS Gettering Furnace with Oxygen Monitor, Centorr Associates (1979)]. Similarly, the chlorine, also known to contain water, has been passed over the surface of the reactive metal used to prepare the fluoride, such as zirconium, in the process of generating the desired chloride vapor. The metal reacts with any oxygen present in the incoming stream to form the solid, low vapor pressure oxide or oxychloride; if the oxygen bearing species is water vapor, the hydrogen will be released. Because of the high thermodynamic driving force for the oxide to form under the conditions used, the oxygen will be effectively removed from any future potential reaction.
The third potential source is the reactor itself, from outgassing and leaks. The reactor is basically constructed of graphite internal components, with an external metal atmospheric shell. The graphite itself is not likely to either capture or release oxides after the furnace is brought to temperature since they would readily react to form CO/CO.sub.2, both gases that would pass through the reaction zone and out the exhaust. The possibility of leaks is limited by helium mass spectrometer leak detection under vacuum. All observed leaks, to the 10.sup.-9 std cc/sec limit of the leak detector, are corrected.
This leaves the HF used in the reaction as the most likely source of the water, and it is well recognized that commercially available HF contains water [William Braker and Allen L. Mossman, Matheson Gas Data Book, Fifth Edition, p. 305 (1971)]. Levels of water up to 300 ppm are "typical," basically limited by an azeotrope that occurs between the HF and H.sub.2 O in the distillation process used to purify the material. Success in reducing the water content in HF to a level of about 60 ppm has been achieved by one of the manufacturer's of HF by repeated distillation over an extended period of time. No other procedure, such as a desiccant, has been demonstrated to be feasible, because of (1) the reactivity of HF, and (2) the similarity of the size of the two molecules. The possibility of an approach using a specially prepared resin claims to be possible for reducing the concentration of water vapor to 100 ppb has been suggested, but is unproven [Data Sheet on Epigrade Purifier for Hydrogen Fluoride, Advanced Technology Materials, Inc. (1987)].